QC 

333 
L.3 


UC-NRLF 


773 


George  Davidson 
1825*1911 


Professor  of  Geography 
University  of  California 


THE 


BOLOMETER" 


BY 


PROF.  S.  P.  LANGLEY. 


ALLEGHENY  OBKKBVATOHY,  Pa." 


READ    BEFORE    THE 


AMERICAN  METROLOGICAL  SOCIETY, 


DECEMBER,   1880. 


NEW  YORK : 
PUBLISHED    BY    THE    SOCIETY. 

GREGORY  BROS.,  PRINTERS,  34  CARMINE   ST., 
l88l 


/s-if+Ufi 


THE 


"  BOLOMETER," 


BY 


PROF.  S.  P.  LANGLEY, 

\V 

ALLEGHENY  OBSERVATORY,  Pa. 


READ    BEFORE   THE 

AMERICAN  METROLOGICAL  SOCIETY, 


DECEMBER,  1880. 


NEW  YORK : 

PUBLISHED    BY    THE    SOCIETY. 

GREGORY  BROS.,  PRINTERS,  34  CARMINE  ST., 

l88l. 


THE  BOLOMETER. 

By  Prof.  S.  P.  LANGLEY. 


I  have  been  honored  by  a  request  from  President  BARNARD,  to 
explain  to  the  Metrological  Society  a  new  instrument  for  measuring 
radiant  energy  which  I  have  called  the  Bolometer  (fio\rf  j^trpov). 
The  instrument  has  been  made  in  large  part  at  the  cost  of  the 
American  Academy,  as  administrators  of  Count  Rumford's  be- 
quest, and  for  a  more  detailed  account,  reference  may  be  made 
to  a  paper  presented  to  them  on  the  8th  December,  1880,  which 
will  appear  in  their  printed  transactions. 

The  thermometer,  it  is  well  known,  w^as  supplanted  as  a  meas- 
urer of  minute  quantities  of  evanescent  heat,  nearly  half  a  cen- 
tury since,  by  the  Thermo-pile  of  Nobili,  which  in  Melloni's 
hands  so  enlarged  our  knowledge  of  radiant  energy.  During 
this  lapse  of  almost  fifty  years,  no  very  essential  addition  or  im- 
provement has  been  made  in  the  pile.  We  have  better  galvano- 
meters, but  we  use  them  to  record  the  indications  of  the  same 
instrument  that  was  employed  two  generations  ago.  Meantime  I 
numerous  very  important  new  questions  are  presenting  them- 
selves, which  science  cannot  answer  ;  as  in  this  field  of  radiant 
energy,  experiment  has  not  kept  pace  with  the  advance  of  opinions, 
apparently  theoretically  just,  but  as  yet  unverified  and  unverifi- 
able  by  the  final  appeal  to  fact.  Of  the  more  important  of 
these  questions,  none  compares  in  consequence,  with  that  of  the 
distribution  of  energy  in  the  solar  spectrum,  for  this  means  a 
knowledge  of  the  distribution  of  the  total  energy  by  which  we 
ourselves,  and  all  animated  nature,  exist  and  act.  The  only 
means  in  any  sense  trustworthy,  of  learning  the  laws  of  this  dis- 
tribution, is  by  direct  measurement  of  it  in  the  spectrum,  where 
it  appears  as  heat ;  for  our  interpretation  of  it  as  light,  or  even  as 


chemical  action,  does  not  yet  admit  of  being  stated  in  units  of 
force,  with  like  precision.  The  heat  however,  even  in  the  pris- 
matic spectrum,  is  almost  immeasurebly  slight,  and  in  the  pure 
or  diffraction  spectrum  it  has  hitherto  been  found  wholly  so. 
Measures  in  the  prismatic  spectrum,  are  liable  to  gross  error  ; 
measures  in  the  diffraction  spectrum  would  be  precious  as  com- 
paratively exempt  from  these,  but  have  been  found  beyond  our 
reach. 

After  spending  a  long  time  in  the  apprenticeship,  I  grew  fa- 
miliar with  the  numerous  precautions  needful  in  using  the  ther- 
mopile, on  excessively  feeble  radiations,  and  made  successive 
attempts  to  measure  the  heat  in  the  diffraction  spectrum  with  the 
most  delicate  procurable  apparatus,  and  with  all  the  aid  sug- 
gested by  some  years'  experience  in  this  special  kind  of  research. 
I  could  not  however  natter  myself  with  the  belief  that  I  had  suc- 
ceeded where  others  had  failed.  I  obtained  evidences  of  heat  in 
different  parts  of  the  diffraction  spectrum,  it  is  true,  and  to  some 
measurable  extent,  but  I  could  not  feel  that  I  had  measured  in 
any  proper  sense,  and  I  became  convinced  that  accurate  meas- 
urement was  impossible  without  more  delicate  and  also  more 
accurate  apparatus. 

This  has  been  the  cause  of  my  devoting  nearly  a  year's  experi- 
ment toward  the  construction  of  a  sort  of  meter  of  minute  amounts 
of  radiant  "  energy,"  (or  perhaps  I  should  here  say  of  radiant 
" heat.")  I  insist  on  the  word  " meter"  because  I  have  not  tried 
to  make  a  thermoscope,  or  indicator,  so  much  as  a  measurer, 
and  because  I  believe  I  could  have  made  a  far  more  sensitive 
instrument  with  a  tithe  of  the  labor,  had,  I  not  always  kept  in 
view  the  need  of  this  quality  of  strict  proportionality  between 
effect  and  cause.  To  illustrate  my  meaning,  we  may  suppose 
the  finger  to  be  applied  to  an  electric  key  which  may  discharge 
a  grain  of  powder,  or  may  explode  a  mine.  In  this  case  there 
is  no  proportionality  between  the  immediate  cause  and  the  final 
effect,  and  this  is  a  rough  but  not  unfair  illustration  of  the 
principle  of  the  thermoscopic  class  of  instruments,  which  is 
here  rejected.  I  sought  my  analogy  rather  in  the  pressure  of 
this  finger  against  the  resistance  of  a  spring  valve  in  a  steam 


3 

engine,  where  an  enormous  but  definitely  graduated  power 
may  be  released  for  each  degree  of  pressure  exercised  at  the 
valve,  and  the  power  of  the  finger  be  multiplied  a  million- fold 
with  a  constant  proportionality  between  the  cause  and  the 
result.  I  have  worked  by  a  method  which  has  been  employed 
by  Siemens,  and  by  perhaps  others,  for  other  purposes,  but 
which  I  believe  there  has  been  no  previous  application  to  the 
present  use. 

If  a  wire  conveying  an  electric  current  be  warmed,  less  elec- 
tricity flows  through  it  than  before.  If  two  such  wires,  carrying 
equal  currents,  meet  in  a  suitable  galvanometer,  the  needle,  soli- 
cited in  opposite  ways  by  equal  forces,  remains  still.  Now  if  one 
wire  only  be  warmed,  the  current  in  one  wire  is  diminished  while 
the  other  flows  as  before.  The  needle  is  then  deflected  by  a  force 
equal  to  the  difference  of  the  two  currents,  and  proportionate  at 
the  same  time  to  the  power  of  the  battery  and  to  the  (possibly 
very  feeble)  radiation  which  warmed  the  wire,  and  which  thus 
has  been  employed  to  modulate  a  force  enormously  greater  than 
its  own,  which  may  be  in  one  sense  said  to  be  magnified  in  pro- 
portion. The  analogy  with  the  finger  and  throttle-valve  is  not 
far  to  seek,  and  the  principle  thus  stated  is  not  hard  to  grasp, 
but  the  application  in  the  concrete  working  tool  is  difficult.  I 
have  not  yet  resolved  all  difficulties  by  any  means,  but  as  I  have 
brought  the  new  apparatus  to  the  point  where  it  is  a  real  meas- 
urer of  radiant  heat,  more  delicate,  and  I  believe  more  accu- 
rate than  the  thermopile,  I  feel  warranted  in  describing  it  even 
in  its  actual  condition  of  progress.  I  should  first  observe,  that 
since  it  is  by  a  changed  resistance,  that  we  work,  we  should 
have  a  large  part  of  the  resistance  of  the  circuit  in  the  small  part 
on  which  the  exciting  ray  falls,  and  that  to  enable  it  to  receive 
and  part  with  its  heat  rapidly,  the  conducting  wire  should  in  this 
portion  be  laminated,  so  as  to  present  a  greatly  increased  surface 
with  the  same  cross  section.  We  observe  also,  that  we  cannot 
use  unlimited  battery-power  on  account  of  the  undue  heating, 
(of  the  portion  of  the  circuit  exposed  to  radiation)  by  the  battery- 
current  itself,  an  effect  which  must  always  be  borne  in  mind. 
We  may  for  instance  choose  if  we  please,  between  having  this 
part  of  the  circuit  possess  a  resistance  of  a  ohms,  and  convev  a 


current  of  b  webers,  or  possess  a  resistance  of  a  n  ohms  convey- 
ing a  current  of  -  webers.  If  n  is  a  considerable  number  the 
latter  construction  will  generally  be  much  more  efficient  but 
also  far  more  difficult.  "We  can  obtain  ten  (or  twenty)  times  the 
resistance  in  the  same  area  by  making  the  laminated- portion  in 
ten  (or  twenty)  parallel  contiguous  strips,  and  though  this  latter 
construction  is  mechanically  more  difficult,  it  has  been  adopted 
in  the  instruments  actually  in  use.  Their  (theoretical)  disposi- 
tion is  as  follows  : 


The  current  from  the  battery  divides  at  A  into  two  ;  one  branch 
containing  a  number  of  parallel  strips  (  a)  the  other  a  similar 
number  (ft),  (  a  and  ff  are  in  practice  always  close  together)  and 
these  evidently  form  two  of  the  arms  of  a  Wheatstone  Bridge. 
(G)  is  a  very  sensitive  galvanometer  which  may  be  at  any  dis- 
tance from  ex  and  ft.  \r\  is  a  resistance  box  introduced  in  the 
circuit  to  enable  us  to  balance  the  galvanometer  currents  in  spite 
of  any  minute  inequality  in  <x  and  ft.  The  currents  reunite  at 
^  and  return  to  the  battery. 

In  the  actual  construction  (as  arranged  for  a  high  resistance 
and  small  current)  the  parts  <x,  ff}  are  formed  of  strips  of  metals 
reduced  by  rolling  or,  by  chemical  or  electrical  deposition,  to 
extreme  thinness.  The  metals  chiefly  experimented  with  have 
been  gold,  copper,  tin,  iron,  steel,  platinum,  and  palladium,  and 
of  these  the  latter  three  give  most  promise.  One  great  difficulty 
has  been  to  get  them  thin  enough  by -rolling,  as  the  processes  of 
the  goldbeater  will  not  answer.  By  the  help  of  Messrs.  Miller, 
Barr  and  Parkin,  of  Pittsburgh,  who  have  furnished  steel  from 
their  mills  in  sheets  only  "om05  thick  and  by  the- aid  of  a  working 
jeweller  of  Pittsburgh,  who  has  re-rolled  these  with  special 


5 


treatment,  I  have  obtained  a  sheet  of  steel  finally  of  about 
0.002  thick  or  such  that  over  12000  of  them  laid  one  on  the 
other  would  not  make  up  one  English  inch.  I  have  also  a  speci- 
men of  Platinum  even  finer,  through  the  kindness  of  the  officials 
of  the  II.  S.  Mint  at  Philadelphia;*  and,  I  am  under  continued 
obligations  to  Mr.  Outerbridge  of  that  institution  as  well  as  to 
Prof  A.  W.  Wright,  of  New  Haven. 


m.m, 
0  .5 


m.m. 
0.5 


This  steel,  platinum,   or  palladium  is  cut  in  strips  from 

wide,  m5m5  long  and  "j,™  02  to  "oioo*  thick  and  twenty  such>  strips 
placed  side  by  side  and  occupying  together  an  area  of  J  of  one 
square  centimeter  form  one  arm  of  the  electric  balance.  The 
other  arm  consists  of  a  similar  number  of  similar  strips  disposed 
in  two  systems  one  on  either  side  of  the  first,  thus : 


Fig.  2 


The  figures  are  of  course  merely  explanatory,  and  do  not  illus- 
trate the  actual  form  of  the  working  instrument. 

The  actual  disposition  is  here  only  very  roughly  indicated,  but 
it  will  be  seen  that  though  there  are  really  but  the  two  balance 
or  bridge  arms  a  and  ft  as  in  the  first  figure,  one  of  these 
arms  (ft)  is  itself  made  in  two  parts,  so  as  to  lie  on  both  sides  of 
the  other. 

Now,  let  the  system  just  shown  be  entirely  enclosed  in  a  hol- 
low cylinder 


*  A  small  specimen  of  this  steel  is  transmitted  herewith. 


H  K  having  non-conducting  walls  and  protected 
by  a  diaphragm  cover  d  (which  can  be  opened 
at  pleasure,)  against  radiant  heat,  which  can 
only,  in  any  case,  reach  the  arm  a  by  passing 
down  the  whole  length  of  the  axis  of  the  cylin- 
der. The  double  arm  ftfi  does  not  lie  in  the 
axis  of  the  cylinder,  and  is  permanently  cut  off 
from  all  external  radiation.  The  result  is,  that 
every  change  of  temperature  in  the  environ- 
ment of  the  balance-arms,  a  (3,  affects  them 
|  both  alike.  Whether  the  apparatus  itself  be 
|  .varm  or  cool,  held  in  the  hand,  or  surrounded 
oy  ice,  the  change  of  resistance  is  equal  in  both 
arms,  and  the  galvanometer  needle,  solicited  in 
opposite  ways  by  currents  which  have  remained 
equal  to  each  other,  remains  at  zero.  But  if 
radiant  heat  is  admitted  by  opening  the  dia- 
phragm d,  it  warms  one  of  the  arms  (  a  )  only 
(the  other  ft  being  covered)  and  the  needle  is 
deflected  not  as  in  the  thermopile  by  the  feeble 
energy  residing  in  the  ray,  but  by  the  energy 
of  the  battery  which  this  feeble  ray  modulates. 


The  action  of  this  instrument  is  very  prompt,  the  thin  strips 
taking  up  and  parting  in  less  than  a  single  second  with  the  heat, 
which  would  require  from  five  to  ten  minutes  to  produce  a  like 
result  with  the  pile.  The  amount  of  energy  which  may  thus 
be  measured  is  surprisingly  small.  I  believe  that  a  change  of 
temperature  of  1 0  010  0  0  of  one  degree  centigrade  in  the  strips, 
can  be  indicated  on  the  galvanometer,  and  I  compute  (very 
roughly)  that  a  beam  of  solar  or  other  heat  not  so  weak  but  that 
it  would  cause  a  recognizable  change  on  the  galvanometer  -in  one 
second,  would  yet  be  so  small  that  if  let  fall  on  a  kilogramme  of  ice 
at  0°C  it  would  be  over  1000  years  in  melting  it ! 

It  will  be  remembered  that  in  dealing  with  amounts  of  energy 
only  less  minute  than  this,  our  instrument  is  not  merely  an  indi- 
cator but  a  measurer. 


Allegheny  Observatory, 

Allegheny,  Pa. 

December  23,  1880. 


Cr^\ 


Perhaps  the  best  evidence  of  the  utility  of  such  a  capacity  of 
minute  measurement,  is  in  the  fact  that  the  distribution  of  energy 
in  the  diffraction  spectrum  is  now  at  last  being  determined  by 
it.  The  detailed  results  must  be  looked  for  elsewhere,  but  they 
promise  to  materially  affect  our  present  received  opinions  on 
some  points  not  merely  of  high  theoretical  but  of  high  practical 
importance.  Among  the  theoretical  ones,  I  will  barely  mention, 
that  the  representations  of  the  three  curves  for  heat,  light  and 
actinism,  given  in  the  text  books,  are  proven  experimentally  to 
be  wholly  misleading.  There  is  here  no  evidence  that  any  solar  > 
energy  as  received  at  the  earth's  surface  increases  and  then  di- 
minishes, in  the  upper  part  of  the  spectrum,  as  ordinarily  fig- 
ured by  the  so-called  '  actinic'  curve  ;  and  the  curves  of  "  heat" 
and  "  light"  approach  each  other.  We  thus  contribute  to  the 
experimental  demonstration  of  a  great  generalization  of  modern 
physics  i.e.  that  "heat,"  "light"  and  "actinism"  are  not  entities, 
but  names  given  to  different  effects  of  one  and  the  same  solar 
energy.  We  do  very  much  more,  for  we  indicate  the  real  distri- 
bution of  this  energy,  and  are  led  to  new  knowledge,  of  conse- 
quence in  its  bearings  on  the  science  of  meteorology,  and  mat- 
ters of  immediate  practical  importance.  I  here,  however,  am 
describing,  not  these  results  (which  I  will  not  enlarge  on  further), 
but  the  instrument  for  obtaining  them,  which  I  trust  will  shortly 
be  ready  for  the  use  of  others  who  may  wish  to  try  it,  and  which 
I  hope  will  prove  to  be  what  I  have  specially  tried  to  make  it — 
a  useful  working  tool  to  the  physicist  in  the  degree  that  it  is  a 
real  METER  of  radiant  energy. 


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